Tools having a square and level in combination are known in the prior art. In U.S. Pat. No. 4,910,876, issued on Mar. 27, 1990, Charmell teaches the use of an arm which can be slideably attached to an ordinary carpenter's level to form a square. Another version of a level with an arm slideably mounted thereon is disclosed by McSorley, in U.S. Pat. No. 4,607,437, issued on Aug. 26, 1986.
Moreover, the benefits of having an extensible level have been widely recognized in the prior art. In U.S. Pat. No. 4,928,395 issued on May 29, 1990, Good discloses such a level having two extenders slideably attached to a center section. Other slideably extensible levels are disclosed by Youmarts, U.S. Pat. No. 4,733,475, issued on Mar. 29 1988; Gould, U.S. Pat. No. 4,503,623, issued Mar. 12, 1985; and Drumright, U.S. Pat. No. 4,862,595, issued Sep. 5, 1989.
Nevertheless, the only prior art device which can readily be converted from an elongated carpenter's rule to a square and vice versa is a foldable level disclosed in U.S. Pat. No. 4,327,501, issued in 1982. There Hurt teaches a tool having an arm which can be pivotted into a position in which the arm projects at 90 degrees relative to the longitudinal centerline of the level.
In the prior art, the installation of dry wall or plywood panel required the use of three instruments: a square, a ruler and a level to determine the proper size and location of plumbing and electrical outlets. Before installing wall panel, for example, a carpenter would need to establish the location of a particular outlet with respect to some fixed point on the panel by measuring certain horizontal and vertical distances accurately. The present method of establishing such a location :includes juggling, often in combination, a level, a ruler and a square it, order to make these measurements. Simultaneously juggling these same three instruments has also been a routine part of window and door installation.